Generally a non-contact probe comprises of one or more emitter sources and one or more receivers. An emitter source projects waves, for example light waves, on the object of interest; the receiver(s) comprising for example a camera, capture(s) the returning waves from the object. For instance, if the receiver is a CCD-camera, it will take one or more shots of the object. By moving the object relative to the non-contact probe or vice versa, shots of the complete object can be taken, i.e. the object is being scanned. Each shot represents a one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) projection of the object depending on the physical form and working principle of the receiver(s). To obtain accurate 3D points on the surface of the object, relative to a fixed coordinate system, from multiple shots the following system parameters are preferably required:                1. the 3D position of the emitter source(s) relative to the receiver(s);        2. the 3D position and orientation of the emitted waves, relative to the receiver(s), coming from the emitter source(s);        3. the characteristics such as focal point of lenses of the receivers used in the non-contact probe;        4. the dimensions and measuring resolution of the receiver(s); and        5. the 3D position and orientation of the non-contact probe or the object relative to a fixed coordinate system, for each receiver capture.        
The actual values of the parameters 1 to 4 are calculated during the calibration. Some of the actual values of parameter 5 are calculated during the qualification procedure, others are given values readily obtainable from the object-non-contact probe set-up. When these parameters are calculated for the given non-contact probe, the conversion, also called compensation, from receiver readings to accurate 3D points can be performed.
To be able to move the non-contact probe relative to the object, the probe is mounted on a localizer. This localizer can be a structure, portable or non-portable, on which the probe is mounted, like for example a tripod. This localizer can also be a structure with moving axes, motorized or non-motorized, where the probe is mounted on the end of said axes, dependent to all other axes, like for example a robot, a milling machine or coordinate measuring machine (CMM). These last types of localizer can have the possibility to record the position and/or the rotation of the probe.
Some of the state-of-the-art techniques actually calculate parameters 1 to 5 explicitly using dedicated measurements in a sequential manner. Adjusting these parameters is difficult because of their complex interactions and adjustment can be time consuming.
Other state-of-the-art techniques for calibration and qualification do not calculate these parameters explicitly. In the first step, the receiver readings are first transformed in one, two or three dimensions, depending on whether the receiver itself is capable of measuring one, two or three dimensions. In this calibration step a receiver reading in receiver units is transformed to a point in e.g. SI units. The calibration can take several phenomena into account; for example, correcting the perspective since the object is not always parallel to the receiver or scaling correctly the readings. Finally, it must model correctly systematic reading errors in the receiver(s)—which is a complex operation.
The second step, the qualification, consists of determining the accurate position and orientation of the non-contact probe relative to a fixed coordinate system. The qualification procedure is generally performed by the end-user using special artifacts or other measuring equipment. The qualification procedure often requires an essential manual alignment of the artifact/measuring equipment with the non-contact probe.
Both steps, calibration and qualification, are based on some parameters that are tuned or optimized to obtain the 3D points accurately. Most state of the art algorithms determine the values of the parameters for the two steps separately, usually scanning different artifacts with known features, dimensions, etc. A method to generate the parameters with an integrated approach using one single artifact and a general function to convert directly a receiver reading into a three dimensional point has been recently disclosed (EP 1 361 414). This function corresponds to a sequence of calibration and qualification and, therefore, depends on many parameters similar to the ones used in the classical de-coupled technique. By expressing the conversion with one function, all interactions between the parameters are considered. Also, if the object being measured is known, the influence of a single parameter can directly be measured in terms of accuracy of the resulting 3D point.
Measurement using such technique relies on accurate determination of the probe position and/or orientation by the localiser for a given probe reading. Any error or lack of accuracy leads to a less precise calibration and qualification, and consequently costly readjustment. When taking measurements, the frequency of the receiver readings and the frequency of localizer readings rarely match or, the frequencies match but the phases are different. For example, a continuously moving probe may generate a high number of probe readings during movement, but this may not be matched by the same number of localizer readings in a given time period. Alternatively, the same numbers of readings are performed but a delay occurs. The difference is due to incompatibility between the electronics of the detector and localizer which are not synchronised. The result is that for each receiver reading there is commonly no corresponding localizer reading. The accuracy and precision of the calibration is consequently reduced. To overcome the problem, the electronic circuitry of both the detector and the localizer may be adapted to provide reading information at the same frequency and in synchronization. However, the adapted circuitry must be specialised and bespoke according to the output type of the respective devices. Should the same localizer be used with several different probes, electronic synchronization would entail a corresponding number of sets of adapted circuitry, so increasing costs. In more complex situations, a receiver obtains multiple readings in a periodic manner, each reading provided with a time stamp but the precise time stamp of each reading in one cycle cannot be synchronised. The synchronization with the localizer data cannot even be obtained with adapting hardware circuitry.
The present invention provides an adaptation to the technique described in (EP 1 361 414). It overcomes the problem of asynchronous data in the probe and localizer by mathematically identifying the time stamp of every receiver reading using a synchronization model with a few parameters and interpolating the corresponding localiser data for this time stamp. The parameters of the synchronization model are computed simultaneously during calibration and qualification.